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SIGACTION(2)                BSD System Calls Manual               SIGACTION(2)

     sigaction -- software signal facilities

     Standard C Library (libc, -lc)

     #include <signal.h>

     struct  sigaction {
             union __sigaction_u __sigaction_u;  /* signal handler */
             sigset_t sa_mask;               /* signal mask to apply */
             int     sa_flags;               /* see signal options below */

     union __sigaction_u {
             void    (*__sa_handler)(int);
             void    (*__sa_sigaction)(int, struct __siginfo *,
                            void *);

     #define sa_handler      __sigaction_u.__sa_handler
     #define sa_sigaction    __sigaction_u.__sa_sigaction

     sigaction(int sig, const struct sigaction *restrict act, struct sigaction *restrict oact);

     The system defines a set of signals that may be delivered to a process.  Signal delivery resembles the
     occurrence of a hardware interrupt: the signal is normally blocked from further occurrence, the current
     process context is saved, and a new one is built.  A process may specify a handler to which a signal is
     delivered, or specify that a signal is to be ignored.  A process may also specify that a default action
     is to be taken by the system when a signal occurs.  A signal may also be blocked, in which case its
     delivery is postponed until it is unblocked.  The action to be taken on delivery is determined at the
     time of delivery.  Normally, signal handlers execute on the current stack of the process.  This may be
     changed, on a per-handler basis, so that signals are taken on a special signal stack.

     Signal routines normally execute with the signal that caused their invocation blocked, but other sig-nals signals
     nals may yet occur.  A global signal mask defines the set of signals currently blocked from delivery to
     a process.  The signal mask for a process is initialized from that of its parent (normally empty).  It
     may be changed with a sigprocmask(2) call, or when a signal is delivered to the process.

     When a signal condition arises for a process, the signal is added to a set of signals pending for the
     process.  If the signal is not currently blocked by the process then it is delivered to the process.
     Signals may be delivered any time a process enters the operating system (e.g., during a system call,
     page fault or trap, or clock interrupt).  If multiple signals are ready to be delivered at the same
     time, any signals that could be caused by traps are delivered first.  Additional signals may be pro-cessed processed
     cessed at the same time, with each appearing to interrupt the handlers for the previous signals before
     their first instructions.  The set of pending signals is returned by the sigpending(2) system call.
     When a caught signal is delivered, the current state of the process is saved, a new signal mask is cal-culated calculated
     culated (as described below), and the signal handler is invoked.  The call to the handler is arranged
     so that if the signal handling routine returns normally the process will resume execution in the con-text context
     text from before the signal's delivery.  If the process wishes to resume in a different context, then
     it must arrange to restore the previous context itself.

     When a signal is delivered to a process a new signal mask is installed for the duration of the process'
     signal handler (or until a sigprocmask(2) system call is made).  This mask is formed by taking the
     union of the current signal mask set, the signal to be delivered, and the signal mask associated with
     the handler to be invoked.

     The sigaction() system call assigns an action for a signal specified by sig.  If act is non-zero, it
     specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask to be used when delivering the
     specified signal.  If oact is non-zero, the previous handling information for the signal is returned to
     the user.

     Once a signal handler is installed, it normally remains installed until another sigaction() system call
     is made, or an execve(2) is performed.  A signal-specific default action may be reset by setting
     sa_handler to SIG_DFL.  The defaults are process termination, possibly with core dump; no action; stop-ping stopping
     ping the process; or continuing the process.  See the signal list below for each signal's default
     action.  If sa_handler is SIG_DFL, the default action for the signal is to discard the signal, and if a
     signal is pending, the pending signal is discarded even if the signal is masked.  If sa_handler is set
     to SIG_IGN current and pending instances of the signal are ignored and discarded.

     Options may be specified by setting sa_flags.  The meaning of the various bits is as follows:

           SA_NOCLDSTOP    If this bit is set when installing a catching function for the SIGCHLD signal,
                           the SIGCHLD signal will be generated only when a child process exits, not when a
                           child process stops.

           SA_NOCLDWAIT    If this bit is set when calling sigaction() for the SIGCHLD signal, the system
                           will not create zombie processes when children of the calling process exit.  If
                           the calling process subsequently issues a wait(2) (or equivalent), it blocks
                           until all of the calling process's child processes terminate, and then returns a
                           value of -1 with errno set to ECHILD.

           SA_ONSTACK      If this bit is set, the system will deliver the signal to the process on a signal
                           stack, specified with sigaltstack(2).

           SA_NODEFER      If this bit is set, further occurrences of the delivered signal are not masked
                           during the execution of the handler.

           SA_RESETHAND    If this bit is set, the handler is reset back to SIG_DFL at the moment the signal
                           is delivered.

           SA_RESTART      See paragraph below.

           SA_SIGINFO      If this bit is set, the handler function is assumed to be pointed to by the
                           sa_sigaction member of struct sigaction and should match the prototype shown
                           above or as below in EXAMPLES.  This bit should not be set when assigning SIG_DFL
                           or SIG_IGN.

     If a signal is caught during the system calls listed below, the call may be forced to terminate with
     the error EINTR, the call may return with a data transfer shorter than requested, or the call may be
     restarted.  Restart of pending calls is requested by setting the SA_RESTART bit in sa_flags.  The
     affected system calls include open(2), read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and
     recvmsg(2) on a communications channel or a slow device (such as a terminal, but not a regular file)
     and during a wait(2) or ioctl(2).  However, calls that have already committed are not restarted, but
     instead return a partial success (for example, a short read count).

     After a fork(2) or vfork(2) all signals, the signal mask, the signal stack, and the restart/interrupt
     flags are inherited by the child.

     The execve(2) system call reinstates the default action for all signals which were caught and resets
     all signals to be caught on the user stack.  Ignored signals remain ignored; the signal mask remains
     the same; signals that restart pending system calls continue to do so.

     The following is a list of all signals with names as in the include file <signal.h>:

     NAME            Default Action          Description
     SIGHUP          terminate process       terminal line hangup
     SIGINT          terminate process       interrupt program
     SIGQUIT         create core image       quit program
     SIGILL          create core image       illegal instruction
     SIGTRAP         create core image       trace trap
     SIGABRT         create core image       abort(3) call (formerly SIGIOT)
     SIGEMT          create core image       emulate instruction executed
     SIGFPE          create core image       floating-point exception
     SIGKILL         terminate process       kill program
     SIGBUS          create core image       bus error
     SIGSEGV         create core image       segmentation violation
     SIGSYS          create core image       non-existent system call invoked
     SIGPIPE         terminate process       write on a pipe with no reader
     SIGALRM         terminate process       real-time timer expired
     SIGTERM         terminate process       software termination signal
     SIGURG          discard signal          urgent condition present on socket
     SIGSTOP         stop process            stop (cannot be caught or ignored)
     SIGTSTP         stop process            stop signal generated from keyboard
     SIGCONT         discard signal          continue after stop
     SIGCHLD         discard signal          child status has changed
     SIGTTIN         stop process            background read attempted from control terminal
     SIGTTOU         stop process            background write attempted to control terminal
     SIGIO           discard signal          I/O is possible on a descriptor (see fcntl(2))
     SIGXCPU         terminate process       cpu time limit exceeded (see setrlimit(2))
     SIGXFSZ         terminate process       file size limit exceeded (see setrlimit(2))
     SIGVTALRM       terminate process       virtual time alarm (see setitimer(2))
     SIGPROF         terminate process       profiling timer alarm (see setitimer(2))
     SIGWINCH        discard signal          Window size change
     SIGINFO         discard signal          status request from keyboard
     SIGUSR1         terminate process       User defined signal 1
     SIGUSR2         terminate process       User defined signal 2

     The sa_mask field specified in act is not allowed to block SIGKILL or SIGSTOP.  Any attempt to do so
     will be silently ignored.

     The following functions are either reentrant or not interruptible by signals and are async-signal safe.
     Therefore applications may invoke them, without restriction, from signal-catching functions:

     Base Interfaces:

     _exit(), access(), alarm(), cfgetispeed(), cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(),
     chmod(), chown(), close(), creat(), dup(), dup2(), execle(), execve(), fcntl(), fork(), fpathconf(),
     fstat(), fsync(), getegid(), geteuid(), getgid(), getgroups(), getpgrp(), getpid(), getppid(),
     getuid(), kill(), link(), lseek(), mkdir(), mkfifo(), open(), pathconf(), pause(), pipe(), raise(),
     read(), rename(), rmdir(), setgid(), setpgid(), setsid(), setuid(), sigaction(), sigaddset(),
     sigdelset(), sigemptyset(), sigfillset(), sigismember(), signal(), sigpending(), sigprocmask(),
     sigsuspend(), sleep(), stat(), sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(),
     tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(), umask(), uname(), unlink(), utime(), wait(),
     waitpid(), write().

     Realtime Interfaces:

     aio_error(), sigpause(), aio_return(), aio_suspend(), sem_post(), sigset().

     ANSI C Interfaces:

     strcpy(), strcat(), strncpy(), strncat(), and perhaps some others.

     Extension Interfaces:

     strlcpy(), strlcat().

     All functions not in the above lists are considered to be unsafe with respect to signals.  That is to
     say, the behaviour of such functions when called from a signal handler is undefined.  In general
     though, signal handlers should do little more than set a flag; most other actions are not safe.

     Also, it is good practice to make a copy of the global variable errno and restore it before returning
     from the signal handler.  This protects against the side effect of errno being set by functions called
     from inside the signal handler.

     The sigaction() function returns the value 0 if successful; otherwise the value -1 is returned and the
     global variable errno is set to indicate the error.

     There are three possible prototypes the handler may match:

           ANSI C:
                  void handler(int);

           POSIX SA_SIGINFO:
                  void handler(int, siginfo_t *info, ucontext_t *uap);

     The handler function should match the SA_SIGINFO prototype if the SA_SIGINFO bit is set in flags.  It
     then should be pointed to by the sa_sigaction member of struct sigaction.  Note that you should not
     assign SIG_DFL or SIG_IGN this way.

     If the SA_SIGINFO flag is not set, the handler function should match either the ANSI C or traditional
     BSD prototype and be pointed to by the sa_handler member of struct sigaction.  In practice, FreeBSD
     always sends the three arguments of the latter and since the ANSI C prototype is a subset, both will
     work.  The sa_handler member declaration in FreeBSD include files is that of ANSI C (as required by
     POSIX), so a function pointer of a BSD-style function needs to be casted to compile without warning.
     The traditional BSD style is not portable and since its capabilities are a full subset of a SA_SIGINFO
     handler, its use is deprecated.

     The sig argument is the signal number, one of the SIG... values from <signal.h>.

     The code argument of the BSD-style handler and the si_code member of the info argument to a SA_SIGINFO
     handler contain a numeric code explaining the cause of the signal, usually one of the SI_... values
     from <sys/signal.h> or codes specific to a signal, i.e. one of the FPE_... values for SIGFPE.

     The uap argument to a POSIX SA_SIGINFO handler points to an instance of ucontext_t.

     The sigaction() system call will fail and no new signal handler will be installed if one of the follow-ing following
     ing occurs:

     [EFAULT]           Either act or oact points to memory that is not a valid part of the process address

     [EINVAL]           The sig argument is not a valid signal number.

     [EINVAL]           An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

     [EINVAL]           An attempt was made to set the action to SIG_DFL for a signal that cannot be caught
                        or ignored (or both).

     The sigaction() system call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1'').  The
     SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as are the signals, SIGTRAP, SIGEMT, SIGBUS,
     available on most BSD-derived systems.  The SA_NODEFER and SA_RESETHAND flags are intended for back-wards backwards
     wards compatibility with other operating systems.  The SA_NOCLDSTOP, and SA_NOCLDWAIT flags are featur-ing featuring
     ing options commonly found in other operating systems.

     kill(1), kill(2), ptrace(2), sigaltstack(2), sigblock(2), sigpause(2), sigpending(2), sigprocmask(2),
     sigsetmask(2), sigsuspend(2), sigvec(2), wait(2), fpsetmask(3), setjmp(3), siginterrupt(3),
     sigsetops(3), ucontext(3), tty(4)

BSD                           September 18, 2008                           BSD

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